Heat-treatment of copper-silver binary alloys



Sept. 11, 1951 A. w. HODGE 2,567,560

HEAT TREATMENT OF COPPER-SILVER BINARY ALLOYS Filed May 6, 1948 2Sheets-Sheet 1 FIGURE 1 Gu l 6 PER CENT SILVER FIGURE 3 TENSILE TENSILESTRENGTH IN lOOO RSI.

CONDUCTIWTY PER CENT IACS 5 6 7 PER CENT SILVER IN COPPER INVENTOR AllenW. Hodge ATTORNEY Sept. 11, 1951 A. w. HODGE I HEAT TREATMENT OFCOPPER-SILVER BINARY ALLOYS 2 Sheets-Sheet 2 Filed May 6, 1948 FIGURE 2D E A E R T m E H .EE 5%. o mv o 00 0 mm ne 8 mm 009 :BEEW 552E TREATEDO 31] 6M 74.I 84.5 88.5 94.0 96.l 9Z4 98.4 99.0

REDUCTION IN AREA, PER CENT INVENTOR Allen W. Hodge evh 5 ATTORNEYPatented Sept. 11, 1951 HEAT-TREATMENT OF COPPER-SILVER BINARY ALLOYSAllen W. Hodge, Columbus-,Ohio, assignor, by mesneassignments, toBattelle. Development Corporation,..Columbus, Ohio, a,- corporation ofDelaware Application'May. 6, 1948, Serial No. 25,509

2 Claims; (Cl. 14812.7)

This invention relates-to a method of-processing binary alloys of copperand silver andto the product thereby produced. The present 7 inventionis particularly directed to the processing of the low-silver alloyswherein the silver does not exceed approximately 8 Binaryalloys ofcopper and silver in which copper is the'base' -metal have been knownfor H some time, but exist primarilyin the laboratory due 'to thefactthat they arenot characterized bysufiiciently outstanding properties tooffset the explicit expense of their production. These binary alloys,wherein the silver is present in amounts under 8%, do not comparefavorably to the much'i cheaper metal copper. These alloys have aconductivity,'when' annealed, of about 90 of thatofpure copper;only'little'higher tensile strength and a lower'elongation:Consequently, very little work has been done with these alloys,particularly because preliminary processingiat tempts Were discouragedby the discovery that, while hardness increased after an age-hardeningheat treatment, the tensile strength'wentdown.

The primary object of the present invention is to i provide a methodof.processing silvercopper alloys containing lessthan. about 8% of silverso as to produce aproduct having unusually high conductivity combinedwithiexcellent tensile strength.

Another object of this invention is to provide a method for aging andcold working a silvercopper binary alloy having a copper base whereinthe. aging time is reduced to. less than onehalffhour.

Other objects and advantages of the present invention will becomeapparent from the following detailed description thereof when read inconjunction with: the accompanying drawings in which:

Figure l is agphase diagram of the copper-rich portion of thecopper-silver system,

Figure 2 is a graph upon which curves are plotted to show the effect ofaging and cold drawing upon the, properties of aparticular silver-copperwire, and

Figure 3 graphically illustrates the effect of variations of the silvercontent in the processed alloy.

It has been discovered, as described and claimed in the copendingapplication, Serial No. 25,508, filed May 6, 1948, on Copper-SilverAlloys, now abandoned, that binary alloys of copper and silver can beaged and then coldworked to produce alloys having. unusual and usefulproperties. The present invention lies in the discovery that preliminarycold Working prior to aging'the alloy increases the hardening andstrengthening effect of subsequent cold working o-f' the alloy overwhat-can be otherwise attained Withoutthe preliminary cold workingtreatment."

Furthermore; this preliminary cold Working'step lowers-the agingtemperature required and great 1y diminishes the timenecessary for theaging treatment.

The process comprising the present invention isapplicable to alloyscontaining less than about 8% silver and which are composed of a singlephase (alpha). These alloys fall within the area lying between lines Iand 2 in Figure 1; Line I represents the limit of solid solubility at'any given temperature of silver in copper, whereas line 2 is the solidusline, that is, the line below which the alloy is entirely in the solidform;

singlealpha phase.

Thejun'ction of lines I and 2 in Figure l is designated point fl and isthe-maximum limit of solid solubility of silver in copper. Line 4indicates the temperature at which the eutectic of this systemsolidifies, that is, about 779 G.

Since pointfl indicates the maximum Solid solubility of silver in copperto be about 8%, silver in excess of this amount will producea two phasealloy, to which the present invention is not applicable. It is verydifficult to obtain complete solid solubility of over-7% silver incopper, so'that it" is preferred to restrictthesilver con-- tent to 7 orslightly'lower. The lower limit for silver is about'3%, because verylittle benefit is derived from the present process using-alloys of lowersilver content; The full effectoftheaging step is not found in alloyscontaining as little as 3% silver, so that the lower preferred limit isabout 5% silver.

As the alloy cools down after the copper and silver havebeenintermingled inthe molten state, the copper-rich phase designated alphain Figure l precipitates out first, leaving the alloy which solidifieslast richer in silver. Conse-' quently; the as=cast alloy contains bothalpha and'beta phase and must be solution annealed or solution heattreated to cause it to reform in the single'alpha phase. It is essentialthat the alloy be first put into a single phase by solution heattreatment in order to age it. The step of solution heat treatment iswell known to metallurgists and need not be described here in detailsince it forms only an incidental part of the presentinvention. It hasbeen found, nevertheless, that solution heat treatment at about 745 C.continuedfor two hours and followed by a water quench produces highlysatisfactory results, if repeated often enough. About four of thesesoaking periods have proven to be sufiicient-to put the alloysubstantially or entirely into the Variations in time andtemperaturefand' manner of quench may be made inthis procedurefwithoutin anyway afiecting the present invention. .Itfis only critical that theincrease gradually up to about 75% reduction in 1 area. Since themaximum benefit is derived from this step when reduction ranges from 75%to about 95%, this range is preferred.

The cold worked alloy is then aged by heating the alloy, usually in amolten salt bath or the like, until a marked drop in tensile strength isobtained. This phenomenon, as indicated above, is directly contrary tothe usual age hardening treatment because normally other alloys thatrespond to the aging treatment increase in tensile strength upon aging.The aging effect is the combined product of time, temperature andcomposition, and the aging step itself, as it is practiced herein, isfamiliar to all metallurgists as being common practice in connectionwith a large number of alloys. In the present instance, it may be statedgenerally that temperatures between 350 C. and 500 C. produce excellentresults with the binary copper-silver alloys to which this inventionrelates. The present aging treatment is remarkable, not only as respectsthe softening action above referred to, but as respects the short periodof time required to effectively age this alloy. For example, an agingtreatment of ten to fifteen minutes at about 400 C. produces veryexcellent results. This extremely short aging period is highly importantfrom a commercial standpoint, because it greatly increases the rate ofproduction.

After aging, the alloy is again cold worked until a tensile strength andhardness has been obtained suflicient for the purpose to which the alloyis to be put. The exact extent of this second cold working step willdepend almost entirely upon the end use of the alloy, but normallysufficient cold working is employed to at least give the alloy a tensilestrength greater than that which it had prior to the aging treatment.Actual practice has shown that maximum tensile strengths are obtainedwhen the alloy is cold worked to about 97% reduction in area after theaging treatment. For example, an alloy of 6% to 7% silver, balancecopper, aged 10 minutes to minutes at 400 C. and cold worked before andafter aging in this preferred manner will have a better combination oftensile strength and electrical conductivity than any other known alloy.Substantially the same result will be obtained with an aging time of 5minutes at 425 C.

A graphic illustration of the effect of the treatment defined in thepresent application upon an alloy containing 6.5% silver and the balancecopper is clearly shown in Fig. 2. The tensile strength is plottedagainst the per cent reduction in area on a logarithmic scale, and thealloy was tested in the form of a wire. The annealed alloy, having atensile strength of 45,000 p. s. i., increases in tensile strength as itis cold worked up to the point of 74.1% reduction in area, at whichpoint the alloy was aged. The aged alloy, represented by the dottedline, dropped in tensile strength rapidly from about 89,000 p. s. i. to76,000 p. s. i. upon aging. However, it required less than additionalreduction of area to raise the tensile strength of the heat-treated oraged alloy above that of the unheat-treated alloy represented by thesolid line on Figure 2. Upon further cold working of the heat-treated oraged alloy, it is clearly apparent that this alloy was much moreresponsive to the cold working treatment than the alloy which was notcold worked prior to aging. At 97% additional reduction in area afteraging the alloy attained a tensile strength of 165,000 p. s. i. andexceeded the tensile strength of the unaged alloy by 40,000 p. s.i.

Not only does the present process produce an alloy having greatlyincreased tensile strength, but the conductivity, as indicated in Figure2, is also increased somewhat by the aging treatments. As above pointedout, the amount of increase in conductivity is dependent upon the lengthof the aging treatment. In Figure 2, the aging treatment was 12 minutesat 400 C. in a nitrate salt bath and Was much too short for anysubstantial increase in conductivity to be obtained. This treatment isintended to give maximum strength with an electrical conductivity of atleast 70% of that of pure copper.

In order to better enable those skilled in the art to more readilypractice the present invention, the following examples are set forth:

Example 1 Aging 1 Tensile Conductiv- Time, fi gg Strength, ity percentMinutes p. s. i. IACS Example 2 An alloy of 5% silver and copper wastreated in identical manner to that of Example 1. The wire produced hadthe following properties:

Aging Tensile Conductiv- Time, g g gs Strength, ity, percent Minutes p.s. i. IAOS Example 3 An alloy of 6% silver and 94% copper was treatedidentically to the foregoing examples and had the following properties:

Oonduc Aging Diameter, Tensile tivity, Time, Inches Strength, Per CentMinutes p. s. i. IAGS Example 4 An alloy of 7% silver, 93% copper wastreated as the foregoing examples down to the aging step. This wire wasthen aged at various times and 5 temperatures to obtain the maximumstrength possible at each temperature after a reduction of area of 97%following aging. These times were found to be:

Aging Aging Maximum Conduc- Temper- Time, Tensile tivity,

ature, Maximum Strength, Per Cent O. Strength p. s. i. IACS Minutes 37520 162, 000 70. 400 7. 167, 000 (18. 0 425 3. 0 169, 000 69. 0

In general, as indicated above, the time of heat treatment becomesshorter and more critical as the temperature increases, and the maximumstrength is higher with higher temperatures. Also, it has been foundthat the ductility of the wire is poor at the point of maximum strength,but that nearly as high a strength is obtained With higher electricalconductivity and better ductility is reached after longer aging periods,such as 5 minutes at 425 C., 12 minutes at 400 C., 30 minutes at 375 C.(at 375 C. aging temperature, the ductility is definitely poorer than athigher temperatures).

Example 5 An alloy of 6% silver, 0.1% magnesium, balance copper, wassolution heat treated and quenched from 750 C. in the form of wire of0.1%-inch diameter. This wire was then cold drawn 82% reduction of areaand aged. Following the aging treatment, the wire was again drawn to 97%reduction of area. This wire had the properties noted below:

Aging Conduc Temp tit? start ature, Minuts S i Per Cent 0. A08

Example 6 An alloy of 6.5% silver, 0.02% magnesium, balance copper wassolution heat treated and quenched from 745 C. in the form of inchdiameter rod. This was then drawn to 94% reduction of area, aged 12minutes at 400 C., and then drawn to 97% reduction of area. The wire(.011 inch diameter) had the following properties:

Tensile Conduc- Strength tivity sile strength of the unaged alloy isalso shown for comparison purposes. Optimum properties appear to bereached at about 7% silver, although the tensile strength begins tolevel off at about 6% silver. Alloys containing 4% silver have lowertensile strengths, but have somewhat higher conductivity than alloyscontaining greater amounts of silver. Even the lowest tensile strengthfor 4% silver alloys (138,000 p. s. i.) is an improvement of more than200% over pure copper. This increase in tensile strength is of supremeimportance in producing light weight communication cable.

It is apparent from the above-detailed description that the presentinvention relates to the processing of copper-silver alloys containingfrom V 3 to 8% silver, which comprises cold working the alloy prior toaging as well as after the aging step. By practicing the presentinvention in its preferred embodiment, a product may be produced havinga tensile strength in excess of 160,000 p. s. i. and a conductivitygreater than 70% of that of copper. The combination of these twoproperties provides a product so unusual that the relatively high costof the binary silver-copper alloys is offset thereby.

What is claimed is:

1. A method of processing binary alloys of copper and silver whichcontain at least 3% and not more than 8% silver, which comprisessolution heat treating and quenching the alloy, whereby substantiallyall of the alloy is in the copper-rich single phase, cold working theresulting alloy until a cross-sectional reduction in area of at least75% and not more than is obtained, aging the cold-worked alloy attemperatures ranging from 350 C. to 500 C. for a period of at least fiveminutes and less than thirty minutes, and cold working the aged alloyuntil an additional reduction in cross-sectional area of approximately97% is produced.

2. A binary alloy of copper and silver which contains at least 3 percent and not more than 8 per cent silver, in the treated state, producedby the process comprising solution heat-treating and quenching thealloy, whereby substantially all of the alloy is in the copper-richsingle phase, coldworking the resulting alloy until a cross-sectionalreduction in area of at least 75% and not more than 95% is obtained,aging the coldworked alloy at temperatures ranging from 350 C. to 500 C.for a period of at least five minutes and less than thirty minutes, andcold-working the aged alloy until an additional reduction incross-sectional area of approximately 97% is produced.

ALLEN W. HODGE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,281,691 Hensel et a1 May 5,1942 FOREIGN PATENTS Number Country Date 521,927 Great Britain June 14,1940 OTHER REFERENCES Transactions, A. I. M. E., vol. 99, 1932, pages112-113.

Age Hardening of Metals, by American Society for Metals, pp. 143 and321. 1940.

1. A METHOD OF PROCESSING BINARY ALLOYS OF COPPER AND SILVER WHICHCONTAIN AT LEAST 3% AND NOT MORE THAN 8% SILVER, WHICH COMPRISESSOLUTION HEAT TREATING AND QUENCHING THE ALLOY, WHEREBY SUBSTANTIALLYALL OF THE ALLOY IS IN THE COPPER-RICH SINGLE PHASE, COLD WORKING THERESULTING ALLOY UNTIL A CROSS-SECTIONAL REDUCTION IN AREA OF AT LEAST75% AND NOT MORE THAN 95% IS OBTAINED, AGING THE COLD-WORKED ALLOY ATTEMPERATURES RANGINGNG FROM 350* C. TO 500* C. FOR A PERIOD OF AT LEASTFIVE MINUTES AND LESS THAN THIRTY MINUTES, AND COLD WORKING THE AGEDALLOY UNTIL AN ADDITIONAL REDUCTION IN CROSS-SECTIONAL AREA OFAPPROXIMATELY 97% IS PRODUCED.